Electro-optic communication systems historically have been more suited to digital modulation than analog modulation. This is due in part to the tradeoff between bandwidth and signal-to-noise ratio in optical systems. However, digital encoding is not always practical for conversion of signals with a large number of channels, such as video signals, primarily for economic reasons. By eliminating the need for encoding and decoding, the complexity and cost of analog communication systems can be greatly reduced.
The most obvious analog modulation scheme for optical communication systems is direct laser modulation. An obstacle to the success of such a system is the nonlinear relationship of the intrinsic input current to the output intensity of typical light-emitting devices. Distortion of the signal caused by non-linearities can be a problem, particularly in multi-channel systems where a high degree of linearity is required to prevent inter-channel crosstalk.
External modulation of CW lasers may be more attractive than direct modulation since, without modulation of input current, such lasers possess spectral stability and low relative intensity noise (RIN). External modulation does not rely on the laser input current to output intensity characteristics, but relies on the input-to-output characteristics of the modulator itself. Thus, linear electro-optical waveguide modulators are extremely important for analog optical fiber communication and signal processing applications.
Well known external modulators do not provide an absolute solution, however, since existing modulators possess their own nonlinear characteristics, thus limiting their linear dynamic range in their application in critical areas, such as radar and CATV. Such modulators include Mach-Zehnder interferometers, directional couplers and absorption modulators. A few techniques for enhancing the linearity of external modulators have been reported. Typically, the nonlinear responses of two modulators are combined in parallel, with the nonlinear response of one used to cancel that of the other. Some improvement in linearity has been demonstrated with parallel combination of Mach-Zehnder modulators and with polarization mixing. These parallel schemes, however, not only require precise control over the parameters in one modulator but also simultaneous relative control over the same parameters between the two parallel components. Hence, the controllability and fabrication tolerance requirements of these designs may be difficult to meet.
Designs have been proposed that combine the nonlinear responses of modulators in series. Feeding the output from a Mach-Zehnder into a synchronously-modulated directional coupler has been proposed by Liu et al., "In Search of a Linear Electro-optic Amplitude Modulator", IEEE Photon. Technol. Lett., Vol. 3, pp. 144-146, February, 1991. While such a system provides extended linearity, simultaneous control is still an issue. Further, serial combination of active modulators may limit the bandwidth of the desired linear response.